Magnetic random access memory (MRAM) is a type of non-volatile magnetic memory which includes magnetic memory cells. A typical magnetic memory cell includes a layer of magnetic film in which the magnetization of the magnetic film is alterable and a layer of magnetic film in which magnetization is fixed or “pinned” in a particular direction. The magnetic film having alterable magnetization is typically referred to as a data storage layer, and the magnetic film which is pinned is typically referred to as a reference layer.
A typical magnetic memory includes an array of magnetic memory cells. Word lines extend along rows of the magnetic memory cells, and bit lines extend along columns of the magnetic memory cells. Each magnetic memory cell is located at an intersection of a word line and a bit line. A magnetic memory cell is usually written to a desired logic state by applying external magnetic fields that rotate the orientation of magnetization in its data storage layer. The logic state of a magnetic memory cell is indicated by its resistance state which depends on the relative orientations of magnetization in its data storage and reference layers. A sense amplifier is used to sense the resistance state of a selected magnetic memory cell to determine the logic state stored in the memory cell.
Writing magnetic memory cells to a desired logic state can be unreliable. Manufacturing variations in the dimensions or shapes or in the thicknesses or crystalline anisotropy of the data storage layers of the magnetic memory cells can cause variations across a wafer in the critical switching fields necessary to reliably write selected magnetic memory cells without writing half-selected magnetic memory cells.
Changes in ambient temperature can cause the temperature of the magnetic memory cells to vary which in turn causes the coercivity of the magnetic memory cells to change. The coercivity decreases with increasing temperature resulting in a decrease in the critical switching field. Increasing temperatures can increase the likelihood that either the bit line write field or the word line write field will be high enough to cause half-select switching of magnetic memory cells. Conversely, decreasing temperatures can increase the likelihood that the sum of the bit line write field and the word line write field will not be higher than the critical switching field.
To offset the manufacturing variations and temperature variations, the word and bit line electrical currents can be chosen to have values that are higher than those required under nominal conditions so that the critical switching fields can be reached. This approach can result in unnecessary power consumption by the magnetic memory, especially when the ambient temperature is increased.